let isWholeNumber string1 =
    if (String.forall (fun c -> System.Char.IsDigit(c)) string1) then
        printfn "The string \"%s\" is a whole number." string1
    else
        printfn "The string \"%s\" is not a valid whole number." string1
isWholeNumber "8005"
isWholeNumber "512"
isWholeNumber "0x20"
isWholeNumber "1.0E-5"
isWholeNumber "-20"